1. Dating with Radioactivity
Chapter 12, Section 3

2. Basic Atomic Structure
Each atom has a nucleus containing protons and neutrons and that nucleus is orbited by electrons
Electrons have a negative electrical charge and protons have a positive charge
Neutrons have no charge
The atomic number of an element is the number of protons in the nucleus, atoms of the same element always have the same atomic number
An atom’s mass number is the number of protons and neutrons in a nucleus
The number of neutrons can vary in a nucleus, and these variants, or isotopes, have different mass numbers

3. Radioactivity
The forces that bind protons and neutrons together in a nucleus are usually strong
When nuclei are unstable, they spontaneously break apart, or decay, in a process called radioactivity
Radioactivity – the spontaneous decay of certain unstable atomic nuclei
An unstable radioactive isotope of an element is called the parent
The isotopes that result form the decay of the parent are called the daughter products
Radioactive decay continues until a stable or non-radioactive isotope is formed

4. Common Types of Radioactive Decay

5. Half-Life
Half-Life – the time for one half of the atoms of a radioactive substance to decay to its stable isotope
If the half-life of a radioactive isotope is known and the parent/daughter ratio can be measured, the age of the sample can be calculated

6. Radioactive Decay Curve

7. Concept Check What is a half-life?
The amount of time necessary for one half of the nuclei in a sample to decay to its stable isotope.

8. Radiometric Dating
Radiometric Dating – the procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes
Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs
The products of decay have also been accumulating at a constant rate
As uranium decays, atoms of the daughter product are formed, and measurable amounts of lead eventually accumulate
An accurate radiometric date can be obtained only if the mineral remained in a closed system during the period since its formation
Although the basic principle of radiometric dating is simple, the actual procedure is quite complex

9. Radioactive Isotopes Frequently Used in Radiometric Dating
Radioactive Parent
Stable Daughter Product
Currently Accepted Half-Life Values
Uranium-238
Lead-206
4.5 Billion Years
Uranium-235
Lead-207
713 Million Years
Uranium-232
Lead-208
14.1 Billion Years
Rubidium-87
Strontium-87
47.0 Billion Years
Potassium-40
Argon-40
1.3 Billion Years

10. Concept Check Why is a closed system necessary in radiometric dating?
An accurate radiometric date can be obtained only if the mineral remained in a closed system since its formation.

11. Dating with Carbon-14
Radiocarbon (Carbon-14) Dating – method for determining age by comparing the amount of carbon-14 to the amount of carbon-12
Carbon-14 is continuously produced in the upper atmosphere
It becomes incorporated with carbon dioxide, which is absorbed by living matter
All organisms—including you—contain a small amount of carbon-14; when an organism dies, the amount of carbon-14 gradually decays
By comparing the ratio of carbon-14 to carbon-12 in a sample, radiocarbon dates can be determined
Because the half-life of carbon-14 is only 5730 years, it can be used to date recent geologic events up to about 75,000 years ago
Carbon-14 is a valuable tool to anthropologists, archeologists, and historians

12. Production and Decay of Carbon-14

13. Concept Check What is compared when dating with carbon-14?
The ration of carbon-14 to carbon-12.

14. Importance of Radiometric Dating
Radiometric dating has produced dates for thousands of geologic events in Earth’s history
Rocks formed on Earth have been dated to be as much as 4 billion years old
Meteorites have been dated at 4.6 billion years old
Radiometric dating has supported the ideas of James Hutton, Charles Darwin, and many others who inferred the geologic time must be immense

15. Assignment Read Chapter 12, Section 3 (pg. 347-350)
Do Section 12.3 Assessment #1-6 (pg. 350)